In this code, whenever i tried to type a String of characters like ("Annnnnnnnnn") it is still follows the operation for Addition, what i would like to do is that the user can only enter a Single Character and not a String, if the user tried to enter the required character for the if else condition plus a string of characters , the system should not accept it.

example
user tried to input a string of characters like "Annnn"

the output should be "Invalid"

what would i like is that, the if else condition will only work if the user tried to input a Single Only Character

This is a good start.
But take a closer look at you condition. use the simplest possible expression ( choice.length() is not 1).
Next you have to prevent the rest of the method from beeing executed.
The easiest was is to include it completely in the else path...